function debounce(fnc, delay) {
    let done = 1;		//记录是否可执行
    return function () {
        if(done) {
            fnc.apply(this, arguments)
            done = 0		//执行后置为不可执行
            setTimeout(()=>{		//计时结束后再置为可执行
                done =1
            }, delay)
        }
    }
}
window.onscroll = debounce(showTop, 1000)
